The present disclosure contemplates instrumentation and procedures for achieving spinal fixation and more particularly to a screw driver with a release for driving a bone screw into a vertebra of a patient.
A typical spinal fixation system 10 as shown in FIG. 1 spans between successive vertebrae V of the spine. An elongated member, such as rod 12, extends along the length of the spine and provides an anchor point for connecting each vertebra to the rod. The rod is typically contoured to approximate the normal curvature of the spine for the particular instrumented spinal segments, which may include lordosis or kyphosis. Anchor devices 15 are provided for connecting the vertebral segments to the elongated member. These anchor devices may include hooks, bolts, screws or other means for engaging a vertebra. For the purposes of the present discussion, the anchor device 15 is a bone screw assembly, such as the screw assembly shown in FIG. 2.
The bone engaging fastener or screw assembly 15 includes a shank 16 that carries threads configured to engage vertebral bone. For instance, the fastener is a multi-axial pedicle screw with a shank that is threaded for engagement within the pedicle of the vertebra. The screw assembly further includes a head 16a by which the screw, and ultimately the vertebra, is fastened to the spinal rod 12. In particular, the head 16a supports a yoke 17 that is generally U-shaped to receive the spinal rod therethrough, as depicted in FIG. 2. The rod 12 may be supported in part by a collar 18 mounted over the head 16a of the bone screw. A cap 19 may carry a set screw 20 that locks the rod within the yoke 17 and thus fastens the rod 12 to the bone screw or the set screw 20 may be threadably attached directly to the yoke 17. In the multi-axial bone screw assembly 15 the yoke 17 is articulatingly attached to the threaded bone screw 16, and more specifically to the head 16a of the bone screw, so that the yoke 17 can adopt a range of spherical angles relative to the bone screw. Thus, the yoke can articulate relative to the bone screw fastened in the vertebra so that the slot 42 can be aligned to receive the connecting rod 25.
While in the past spinal fixation systems using multi-axial screws have been implanted in open procedures involving relatively large incisions through the patient's tissue with significant muscle retraction, more recent procedures have been developed to introduce spinal fixation systems in a minimally invasive or percutaneous manner. A screw driver for use in implanting multi-axial bone screws is such procedures is shown and described in commonly-assigned U.S. Pat. No. 8,394,108, issued on Mar. 12, 2013 to Scott McLean and Stephen Seyer, (hereinafter “the 108 Patent”), the disclosure of which is incorporated herein by reference in its entirety. While the screw driver of the '108 Patent is an improvement in the art, it is also desirable to provide further features that aid the surgeon in readily releasing the screw driver instrument from the bone screw after implantation.